Installation guide

14
Reflections
The best possible performance and therefore distance, will be achieved if there is
perfect unobstructed line-of-sight between the two sites ‘A’ and ‘B’. Even in this
situation however there is almost certain to be some degree of reflections from the
ground or nearby structures.
These reflections will have an effect on the signal strength and the final picture quality
obtained. It is useful therefore to understand the basic principals of a radio link and the
effect of reflections. The most common reflection is from the ground although other
structures, buildings etc. will have a similar effect. The reflections from these other
structures are less predictable because they depend on the shape of the structure,
construction material etc.
Consider a radio path between site ‘A’ and ‘B’ with each antenna 10 metres from the
ground. Assuming good line of sight, there will obviously be a path directly between
the two antennas, drawn as a straight line on the diagram. There may also be one or
more paths that are reflected from the ground and reach site ‘B’ indirectly via a point
on the ground, ‘X’. Obviously the distance A-X-B is longer than the direct route A-B.
The receiver at site B will receive a direct signal and a reflected signal via point X.
The effect of this reflected signal depends on its strength and the difference in length
between A-B and A-X-B. If the difference is a multiple of the wavelength
(approximately 12.5cm for 2400) the reflection is 180º out of phase and will reduce the
received signal strength. In fact if both signals were the same strength they would
cancel and no signal would be received.
Because at these frequencies the wavelength is quite short, even a small adjustment
in antenna height can affect the path length of the reflected signal and so change the
received signal strength. By raising the antenna gradually it will cycle through a
regular pattern of in phase and out of phase signals giving a corresponding pattern of
reduced and increased signal strength.